X-ray Mirror

Search

Development of the Highest Energy X-ray Mirror

A new type of X-ray mirror that allows images to be made from higher
energy radiation than ever before has recently completed testing at NASA's
Goddard Space Flight Center, Greenbelt, MD. Previously, these high energy (or
"hard") X-rays could only be detected using large detectors, which do not form
an image. The imaging technique offers great improvements in
sensitivity and resolution, and will allow the direct observation of important
physical processes in such unusual celestial objects as neutron stars and black
holes. Often, these objects are associated with the emission or generation of
high energy X-rays.

Credit: NASA

The mirrors, which utilize a new multilayer coating over foil
reflectors, were found to successfully focus an image over an X-ray energy
range of up to 35 kiloelectron volts (keV) during tests at Goddard from
Aug. 12-20, 1997. An electron volt (eV) is a measure of the energy carried by a
particle of light, called a photon. Photons at visible light energies carry
about 2 - 3 eV, so the new mirrors can reflect light that is about 15,000
times more energetic than visible light. The mirrors are the product of a
collaboration between two teams of researchers, one led by Dr. Peter
Serlemitsos of Goddard and the other by Dr. Koujon Yamashita at the
University of Nagoya, Japan.

The ability to provide true imaging will offer significant
improvements in sensitivity and resolution over current instruments in the
hard X-ray band. Focusing the X-rays from a distant source onto a tiny spot
makes the most of a weak signal, since it is not overwhelmed by background
noise from a much larger area. The signal-to-noise ratio is expected to be
hundreds of times better, resulting in the ability to detect and study distant
sources from which only a small amount of radiation reaches the Earth's
vicinity.

Scientists have long been aware of the advantages of X-ray imagers,
but they have been difficult to build. X-rays have enough energy to pass right
through an ordinary mirror instead of being reflected off it. However,
lower-energy or "soft" X-rays can be reflected if the mirror is stood on
edge so that the incoming radiation just grazes the surface. The sensitivity
of a "grazing-incidence" mirror can be increased by nesting several such
surfaces inside each other like a set of measuring cups. Still, hard X-rays
above about 10 keV have too much energy to be detected by such a system
without the addition of the new multilayer coating. The new coating technology
increases the energy range that the mirrors can reflect to approximately 40
keV.

Hard X-rays are given off as thermal energy from the very hottest
objects, such as the region around black holes and the jets emitted from
active galactic nuclei. Non-thermal emission sources of hard X-rays include the
acceleration of particles in very high magnetic fields, such as those of
neutron stars. Telescopes incorporating the new mirrors will also be able to
observe changes in X-ray emission during the repeating cycles of pulsars, as
well as evidence of the production of heavy elements in supernova
explosions.

It is expected that the new mirrors will be used on the International
Focusing Optics Collaboration for micro-Crab Sensitivity
(InFOCuS), a balloon-borne instrument proposed for launch in 1999. The
micro-Crab, a unit used in X-ray astronomy, is equivalent to one millionth
of the X-ray emission from the much-studied Crab Nebula. The achievement
also represents an enabling technology for the proposed Constellation-X
mission.